So I've only watched the first of the double-header that aired last Sunday. But I called the [SPOILER ALERT] sister thing pretty much from the start since this girl Lin was acting like an annoying little sister from moment one.
Though how and why the Mycroft thing? Maybe that gets answered in the next episode, so I guess I'll find out. I can make a few educated guesses though.
Let's skim the story here. Lin is a real estate agent who uses empty properties to host illegal poker games. One of the poker games gets robbed, but Lin doesn't go to the police because, um, illegal. So this episode begins with Lin coming home to her own apartment and finding a bag of money and getting shot at, all of which sends her scurrying to Sherlock Holmes.
I have to say, more episodes should begin this way: with a client appearing. Just like in the stories. But that's a personal preference.
Since Lin's apartment is a crime scene and also probably not safe, she moves in to the brownstone and "little sistering" begins. Lin tells Watson about how Mycroft Holmes was the love of her life, and that's how she knew about Sherlock, but Watson smells a rat and discovers Lin is really her half-sister. (Lin never knew Mycroft.)
The rest of the plot is NSA involvement and something about Eastern European spies.
Points to this episode for giving Sherlock some of his funniest, driest bits of dialogue. I actually laughed aloud a couple times. More of that, please.
Meanwhile, while Lin is annoying in a sister kind of way, she has the potential to bring interesting new energy to the show . . . assuming she stays around. Elementary has a habit of introducing then releasing characters in a somewhat frustrating way.